1. Technical Field
The present disclosure relates to a method for detecting an object by means of a detection signal supplied by a capacitive touch pad-type proximity sensor. The present disclosure applies mainly, but not exclusively, to capacitive touch pads implementing a charge transfer technique.
2. Description of the Related Art
Such a touch pad is widely used for example in man/machine interfaces for inputting system data and commands. Therefore, such a touch pad is designed to detect and locate a finger of the user on the touch pad, i.e., at a distance of less than a few millimeters from the latter. Some touch pads are transparent and are associated with a screen in devices such as mobile telephones.
FIG. 1 represents a touch pad TS comprising electrodes T1, . . . Tp, R1, . . . Rn having the form of bands, which include electrodes Ti (i being a whole number ranging between 1 and p) disposed in columns and electrodes Rj (j being a whole number ranging between 1 and n) disposed in rows transversal to the electrodes Ti. Generally, only one of the so-called “sending” electrodes is activated at a given instant, and so-called “receiving” electrodes are scanned one after the other or simultaneously to obtain measurements representative of the capacitance of each pair of electrodes comprising the active sending electrode and the scanned receiving electrode. The column electrodes Ti (or row electrodes Rj) are connected as sending electrodes, and the row electrodes Rj (or column electrodes Ti) are connected as receiving electrodes. Using the measurements obtained, the position of an object on the touch pad may be determined, given that the presence of an object on the touch pad can change the capacitance of pairs of electrodes located near the object.
Amongst the capacitance measurement methods suited to touch pads, there are particularly methods based on the measurement of a capacitor charge or discharge time, methods based on the use of a relaxation oscillator, and methods based on the charge transfer principle. The methods using a relaxation oscillator involve generating a signal having a frequency which varies according to the capacitance to be measured, then measuring the frequency of that signal. The methods based on charge transfer involve using a “sampling” capacitor, with a high capacitance compared to the capacitances to be measured, charging the capacitance to be measured, and transferring the charge of the capacitance to be measured into the sampling capacitor, and repeating these charge and transfer operations a certain number of cycles. Certain methods based on charge transfer involve executing a fixed number of charge and transfer cycles, and measuring the voltage at the terminals of the sampling capacitor, which is representative of the capacitance to be measured, at the end of the fixed number of cycles. Other methods based on charge transfer execute charge and transfer cycles until the voltage at the terminals of the sampling capacitor reaches a threshold voltage, the number of cycles thus executed being representative of the capacitance to be measured.
An example of implementation of the method based on charge transfer applied to a touch pad is described in U.S. Pat. No. 6,452,514. FIG. 1 represents a control circuit IOC of the touch pad TS, as described in that document. The circuit IOC comprises input/output ports P0, P1, . . . Pn and output ports Pn+1 to Pn+p. Each input/output port Pj (j being a whole number ranging between 1 and n) is connected to a respective input/output stage of the circuit IOC. Each input/output stage comprises a switch I1 controlled by a signal S1, and a transistor M3 the gate of which is controlled by a signal S3. Each switch I1 comprises a terminal connected to a node common to other input/output stages and a terminal connected to the port Pj and to the drain of the transistor M3. The source of each transistor M3 is connected to the ground. Each output port Pn+i (i being a whole number ranging between 1 and p) is linked to a supply voltage source Vdd of the circuit through a transistor M2 the gate of which is controlled by a signal S2. The port P0 is connected to the drain of a transistor M5 the gate of which is controlled by a signal S5 and the source of which is grounded. The port P0 is also connected to a logic circuit LGC supplying the control signals S1, S2, S3, S5 of each input/output and output stage.
To control the electrodes T1-Tp and R1-Rn, the port P0 is connected to a terminal of a sampling capacitor (Cs) the other terminal of which is connected to the ground. The ports P1 to Pn are connected to the row electrodes R1-Rn, and the ports Pn+1 to Pn+p are connected to the column electrodes T1-Tp.
Each row electrode Rj forms with each of the column electrodes Ti a capacitor the capacitance of which varies particularly according to the proximity of an object to an area in which the row electrode overlaps with the column electrode. The circuit LGC receives numbers (i, j) of a pair of ports to be analyzed Pn+i, Pj to locate an object on the touch pad TS, and supplies a measurement DT representative of the capacitance of the pair of electrodes Ti, Rj connected to the selected pair of ports Pn+i, Pj. The measurement representative of the capacitance of the electrode is obtained according to a number of cycles executed for charging the pair of electrodes and transferring the charge to the sampling capacitor Cs, and to the voltage at the terminals of the capacitor Cs after the number of cycles executed.
The logic circuit LGC manages the control circuit IOC that has just been described in accordance with a sequence of steps summarized in Table 1 below:
TABLE 1PortP0PjPn + iStepS5S1S3S2Description11010Discharge of Cs and Rj20010Dead time30101Connection of Cs to Rj and Ti to Vdd40010Dead time50010Rj on 060010Dead time70010Reading of the charge of Cs
In Table 1 and below, i and j represent whole numbers varying from 1 to p, and from 1 to n, respectively.
The sequence of steps which comprises steps 1 to 7, is executed successively for each port Pj and each port Pn+i, and thus for each pair of electrodes (Ti, Rj) connected to the circuit IOC. During the execution of this sequence, all the switches I1 and transistors M2, M3 of the circuit IOC, the control signals S1, S2, S3 of which are not mentioned in Table 1, remain open or off. Step 1 is an initialization step during which the signals S3, S5 switch on the transistors M3, M5 connected to the ports P0 and Pj, to discharge the capacitor Cs and the selected electrode Rj. The next step 2 is a dead-time step during which all the transistors M2, M3, M5 are off and all the switches I1 are open. In step 3, the switch I1 connected to the port Pi is closed to enable a charge transfer between the electrode Rj and the capacitor Cs. In parallel, the transistor M2 connected to the port Pn+i is switched on to charge the electrode Ti to the supply voltage Vdd. The result is a charge transfer between the electrode Rj and the capacitor Cs. The next step 4 is a dead-time step, identical to step 2. In the next step 5, the transistor M3 connected to the port Pj is switched on to discharge the electrode Rj. The next step 6 is a dead-time step, identical to step 2. In the next step 7, all the switches I1 remain open and only the transistor M3 connected to the port Pj is switched on. The voltage of the port P0, corresponding to the voltage of the capacitor Cs, is then measured.
The execution of steps 3 to 6 is repeated a certain fixed number of cycles. After executing this number of cycles, the voltage of the port P0 is measured. The presence and the position of an object on the touch pad TS is then determined according to the measurements obtained for each pair of electrodes Ti, Rj. In practice, a finger of a user can only be detected and located on the touch pad TS if it is less than a few millimeters from an overlapping area of the electrodes of a pair of electrodes (Ti, Rj).
In the touch pad control mode previously described, the capacitor Cs receives negative charges from the electrode Rj to which it is linked in step 3. The voltages to be measured to locate an object on the touch pad are thus negative, which complicates the measuring circuit. In other words, such voltages cannot be measured with standard microcontrollers comprising analog-digital converters. To overcome this problem, the negative charge stored in the sampling capacitor is limited to a low value below the threshold voltage of protective diodes of the input ports of the microcontroller. This results in a low dynamic range which limits the detection sensitivity.
It may also be useful to integrate a proximity detector into a system such as a mobile telephone, integrating a touch pad of the type described above, to activate or deactivate the system or more generally, to activate or deactivate certain functions of the system. Thus, the proximity detector may be used to detect when the user moves his hand or a finger to a distance of a few centimeters from the system. For example, control keyboard backlighting may be activated when the user moves his hand toward the keyboard. A proximity detector may also be integrated into a mobile telephone to lock a touch-sensitive keyboard and/or put a screen into low-energy mode during a telephone call, when the user moves the telephone close to his ear.
Generally, such a proximity detection is performed using a dedicated electrode, which is large in size similar to a contact-activatable touch pad electrode. In a mobile telephone, such a proximity detection electrode may be disposed around the keyboard and/or the screen. Due to its large dimensions, it may be difficult to integrate such an electrode into a small system. Adding such an electrode also requires providing a dedicated detector circuit, connected to the electrode to produce a proximity detection function based on a signal supplied by the electrode. Therefore, integrating a proximity detection function into a system contributes to increasing the dimensions, the cost and the current consumption of the system.